Because you have to say "з приятелем" if you want to speak correctly. There's a lot of Russian contamination currently in the language, but if you want to learn Ukrainian proper, and not a Russian-Ukrainian pidgin, you need to say приятелем.

November 4, 2016

[deactivated user]

I think Zonia435615's Ukrainian seems to be influenced by Polish more than by Russian.

However, I believe «приятель» is not downright incorrect in Ukrainian, it's just much less common than 'przyjaciel' in Polish. At least it's found in Ukrainian dictionaries. So I think it should be accepted as an alternative.